Electrical musical instrument phase shift vibrato system

ABSTRACT

A vibrato or similar system for musical instruments which has successive phase shift stages in which the degree of phase shift is variable depending upon an input variable modulating voltage at a vibrato frequency. Each of the phase shift stages makes use of a field effect transistor as a variable resistance element and includes a circuit feature which solves the problem of intermodulation distortion normally encountered in such circuits.

United States Patent 1 1 Mathias [54] ELECTRICAL MUSICAL INSTRUMENT PHASE SHIFT VIBRATO SYSTEM [75] lnventor: Robert G. Mathias, Brookfield, 111.

[73] Assignee: Hammond Corporation, Deerfleld,

' Ill.

221 Filed: July 3, 1972 21] Appl. NO.Z 268,696

[52] US. Cl .......84/l.25, 331/47 [51'] Int. Cl. ..'..Gl0h l/02, GlOh 1/04 [58] Field of Search ..84/1.24, 1.25, DIG. 4; 331/46-50, 52, 106; 107 R, 108 R, 108 B,

[ i References Cited UNITED STATES PATENTS 2,916,706 12/1959 Timperman ..84/D1G.4 3,256,380 6/1966 MeinemaetaL. ..84/1.25 3,418,418 12/1968 Wilder .......84/1.25

[ 1' June 5, 1973 3,510,567 5/1970 Fisher ..84/l.25 3,516,318 6/1970 Wayne ..84/1.25 x 3,524,376 8/1970 Heytow ..84/l.25 3,609,204' 9/1971 Peterson ..84/l.25 3,609,205 9 1971 Schwartz et a1. ..84 1.25 3,644,657 2/1972 Miller.... ..s4/l.25

Primary Examiner-Richard B. Wilkinson Assistant Examiner-Stanley J. Witkowskl Attorney-William F. Gradolph and-Howard H.

Rogers, Jr. {57 ABSTRACT A vibrato or similar system for musical instruments which has successive phase shift stages in which the degree of phase shift is variable depending upon an input variable modulating voltage at a vibrato frequency. Each of the phase shift stages makes use of a field effect transistor as a variable resistance element and includes a circuit feature which solves the problem of intermodulation distortion normally encountered in such circuits.

3 Claims, 2 Drawing Figures ELECTRICAL MUSICAL INSTRUMENT PHASE SHIFT VIBRATO SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention Vibrato. systems for electrical musical instruments which produce a cyclical variation in frequency by cyclically shifting the phase of the electrical music signal in response to a cyclically varying voltage at a frequency of the order of '6 Hz. 2. Description of the Prior Art Vibrato which is a cyclical minor variation in the frequency of the musical signal of an electrical musical instrument at a vibrato rate, of the order of 6 Hz, is commonly achieved by cyclically phase shifting the signal. Occasionally, also, such systems are used at a low cyclical rate of the order of l to I be Hz for a celeste type effect. The present invention is also useful for such purpose which is included in the general designation of vibrato systems. Typical methods of accomplishing this phase shift form the subject of US. Pat. Nos. 3,258,519 and 3,325,581 issued to Alan C. Young. In one of the circuits shown in US. Pat. No. 3,258,519, FIG. 3, a variation in the resistance of one leg of a phase splitter circuit progressively phase shifts the signal depending upon the instantaneous value of the resistance. In that patent, the variable resistors take the form of light dependent resistors and the resistance is cyclically varied by varying the. intensity of the light falling upon these resistors. A generally similar system using current sensitive variable reactors forms the subject of Meinema et al. US Pat. No. 3,256,380.

SUMMARY OF THE INVENTION The present system is a generally similar but improved arrangement in that the circuit is transistorized and the variable resistor elements are'field effect transistors. In general, this substitutes for the light dependent resistors, the light source and the motor driven rotary shutter mechanism of the Young patents, or the rather expensive reactor system of the Meinema et al. patent.

The advantages of using field effect transistors rather than light dependent resistors as variable resistance elements are considerable. In addition to the lower cost and greater simplicity of such a system there is the advantage of substantially instantaneous response of resistance change with control voltage change and the avoidance of the hysteresis effect present in light dependent resistors. There is also the advantage of much greater uniformity in the components which is of rather critical importance. I

Light dependent resistors respond rather slowly to change in the light intensity falling upon the element. Typically, large changes in resistance may require from one tenth to one second to stabilize. Furthermore, an increase in the illumination of the LDR will decrease the resistance faster than a decrease in illumination will incrcaseit. The net result of this slow response and the hysteresis effect is that it is difficult to obtain components or devise an LDR system that will respond fully at a vibrato rate, about 6 Hz, and which will have a sinusoidal phase shift versus time characteristic.

The amount of phase shift required for a suitable vibrato effect is rather large, particularly at the higher audio frequencies, of the order of 360 or more. As a practical matter, each stage should not be called upon to shift the phase more than 90 to 100. Multiple stages are, therefore, required and it is important that all stages shift the phase by the same amount. This is not a problem when field effect transistors are used, since these elements have a high degree of uniformity. Light dependent resistors, however, are rather poor in this respect and there is, therefore, the necessity for trimming the units either optically or by means of resistors. This is an expensive requirement and even then, matching the performance of the several stages is seldom completely possible.

The variable reactor system of the Meinema et al. patent has problems of its own upon which it is not necessary to elaborate here, because on the basis of relative cost alone, the present system has obvious advantages which indicate its greater suitability.

A considerable but different difficulty, however, is-

associated with the use of field effect transistors for this purpose as will appear presently in that they inherently BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of a fourstage phase shift vibrato circuit which makes use of the featuresof the present invention; and 7 FIG. 2 is a circuit diagram of one of the phase shift stages, all of which are identical.

DESCRIPTION OF THE PREFERRED EMBODIMENT -Referring to FIG. 1 the music signal, whatever is being played upon the instrument at the moment, enters the circuit at the terminal 10 through the netword comprised of resistors 12 and 14. This network 'may be whatever is required for isolation and generally to adapt the signal to enter the phase shift portion of the circuit. The four stages of phaseshift which are in series are indicated at 18, 20, 22 and 24. Each of these is connected by a lead 26 to a line 28 maintained at approximately +5 v. For convenience in reference, the signal channel, that is, the lead to each, and between each, of the phase shift stages has been given the numeral 30 as has the output lead from the last stage 24.

From the last stage the signal enters an amplifier 32 which may also make use of the +5 v. lead 28. The output of the amplifier is indicated at 34 and will normally be connected to the output circuit of the musical instrument. The output circuit and the amplifier may be conventional and form no part of the present invention. Each of the phase shift stages 18, 20, 22 and 24 and amplifier 32 are also connected to the other side of the circuit indicated by the ground lead 36. An oscillator 37, which may be conventional, supplies a variable voltage modulating signalat a vibrato rate to a lead 40 which is connected through branches 42 to each of the phase shift stages 18, 20, 22 and 24.

All of the phase shift stages 18, 20, 22 and 24 are identical and one of these is shown in detail in FIG. 2. Although four stages are shown, and this may be considered typical, more or fewer stages cam be provided, depending upon the degree of phase shift desired. The input lead 30 is connected through a capacitor 44 to of transistor 46 is connected to the base of transistor 52 and the emitter of transistor 52 is connected to the junction between capacitor 48 and resistor 50. The collector of transistor 52 is connected through a capacitor 56 to the output of the stage at 30. The emitter of transistor 52 is connected to the source of a field effect transistor 58, the drain of which is connected to the outputlead 30. The lead 40 from the vibrato oscillator 38 is connected through branch 42 and a resistor 60 to the gate of the field effect transistor 58 which is also connected through a capacitor 62 to the common point between a pair of series connected resistors 64 and 66. The opposite end of resistor 64 is connected to the source of the field effect transistor 58 and the opposite end of resistor 66 is connected to the drain of the field effect transistor. The gate is biased either negative or positive, depending upon the type, by a connection through a resistor 67 and potentionmeter 69 to a source of appropriate voltage, represented by the terminal 71.

In general, in a phase splitting amplifier of this type, where one output phase of the amplifier is connected through a capacitor to a common output and the other phase is connected through a variable resistor to the common output, the phase of the signal shifts as the resistance of the resistance element is varied essentially for the following reason. The signal voltages at the collector and emitter are substantially 180 out of phase and the phase of the input of the following stage is dependent upon the reactance of the capacity element 56 at any particular frequency and the resistance of the branch through the variable resistor. The amount of phase shift, therefore, assuming that the value of the capacitor remains constant, is dependent upon the resistance in the variable resistance branch. This is a well understood phenomenon and is explained, for instance, in the previously referred to patents.

The field effect transistor at 58 will act as a variable resistor and if its resistance depended solely upon the instantaneous voltage at the gate, which in turn would be the sum of the instantaneous output of the oscillator 38 and the direct current bias voltage, the phase shift of the circuit would take place cyclically at the vibrato rate and ideally there would be no problem.

Unfortunately, for this application, the resistance of the field effect transistor does not depend alone upon the voltage applied to the gate, but rather upon the difference between the voltage at the gate and the voltages at the source and drain. it is, therefore apparent that as the voltages at the source and drain rise and fall due to the music signal applied to the source and conductance through the field effect transistor, this will also affect the resistance through the field effect transistor. The effect of this is to introduce harmonic distortion into the musical signals when the vibrato oscillator voltage is between its high and low peaks. This harmonic distortion of the signal is something that cannot be tolerated if it approaches any appreciable level.

One way of solving this problem is to use a variable modulating voltage at the gate which is high as compared to the signal voltage onthe source. Under these circumstances, the distortion will be of a low order. Thee problem here is that this approach reduces the signal to noise level of the system below an acceptable standard. The solution provided by the present system is to connect the identical resistors 64 and 66 to the source anddrain so that the center point between these resistors is representative of the average value of the instantaneous voltages at the source and drain and then to connect this center point through the capacitor 62 to the gate. The average instantaneous voltage, therefore, as between the source and the drain is introduced at the gate along with the modulating signal and largely cancels the effect of the music signal upon the effective resistance of the field effect transistor so that the resistance of the field effect transistor more truly reflects the modulating voltage only. The result is largely to eliminate the distortion and permit the use of a higher level music signal at the source than would otherwise be feasible.

Representative values for the constants in the illustrative circuit are capacitor 44, 0.01 mf; capacitor 56, 0.0047 mf; capacitor 62, 0.0022 mf; capacitor 48, 220 pf; resistors 54 and 50, l K; resistors60 and 67, 470 K; resistors 64 and 66, 12 K; and resistor 55, 4.7 M.

As stated above, each of the phase shift stages 18, 20, 22 and 24 may have the configuration of the circuit of FIG. 2. Thetotal phase shift will, therefore, be the sum of the phase shift of the individual stages. A convenience of this circuit is that with the exception of the signal channel, all of the connections to the individual stages are in parallel including the connections to the vibrato frequency oscillator 38. This is particularly important if the circuit is packaged as a unitary structure, since it reduces the necessary package terminals to the number required for a single stage.

Although the modulating signal is described as being supplied by a vibrato oscillator operating at a frequency of about 6 Hz, it will be appreciated that for some animation effects it may bedesired to use a more random or other type modulating voltage. Any such variation does not, of course, affect the basic modulating circuit which is the subject of this invention.

I claim:

1. In a phase shift vibrato system for an electrical music signal, a signal input, a signal output, a source of modulating variable voltage at a sub-audible rate, means providing one or more phase shift stages in series between said input and said output, each of said stages comprising an amplifier having an input and a pair of oppositely phased outputs, parallel branches connecting the oppositely phased outputs to a common output for the stage, one of said branches including a series capacitor, the other of said branches including a field effect transistor having a source, a drain and a gate, a connection from said amplifier to said field effect transistor source and a connection from said drain to the common output for the stage, a connection from said source of modualating voltage to said gate, a pair of identical series connected resistors connected between the transistor source and drain, and circuit means including a series capacitor connected from the common point between said resistors to said gate.

2. In a variable voltage controlled phase shift system for an electrical music signal, an amplifier having a signal input and a pair of oppositely phased outputs, a pair of circuits connecting the oppositely phased amplifier outputs to a common output, a field effect transistor having a source, a drain and a gate, one of said pair of means including a series capacitor for applying said average of said voltages to said gate.

3. The system called for in claim 2 in which the circuit means for obtaining the average of the instantaneous voltages comprises a pair of equal value resistors in series connected between the transistor source and drain and a tap at the common point between said resis- IOI'S.

Patent No.

Invent0r( Robert G. 'Mathias June S 1973 r Dated It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

C18 line 58 "modualcing" -'-modul ating- Col. 2, line I 7 line Col. 3, line line line

Col. 4, line Signed and,

(SEAL) Attest:

EDWARD M.FL'ETCHER,JR.

Attest'ing Officer 38, "netword" should be -network- 58, "37" should be -38- "lees" should be --lead--;

34, "180" should be'--l80--;

36 "capacity" should be --capacitor-- 2, "Thee" should be --The-'- should be sealed this 25th da} of September 1973'. 1

RENE D. TEGTMEYER Acting Commissioner of Patents 

1. In a phase shift vibrato system for an electrical music signal, a signal input, a signal output, a source of modulating variable voltage at a sub-audible rate, means providing one or more phase shift stages in series between said input and said output, each of said stages comprising an amplifier having an input and a pair of oppositely phased outputs, parallel branches connecting the oppositely phased outputs to a common output for the stage, one of said branches including a series capacitor, the other of said branches including a field effect transistor having a source, a drain and a gate, a connection from said amplifier to said field effect transistor source and a connection from said drain to the common output for the stage, a connection from said source of modualating voltage to said gate, a pair of identical series connected resistors connected between the transistor source and drain, and circuit means including a series capacitor connected from the common point between said resistors to said gate.
 2. In a variable voltage controlled phase shift system for an electrical music signal, an amplifier having a signal input and a pair of oppositely phased outputs, a pair of circuits connecting the oppositely phased amplifier outputs to a common output, a field effect transistor having a source, a drain and a gate, one of said pair of circuits comprising a connection from said amplifier to said transistor source and a connection from said drain to said common output, the other of said pair of circuits comprising a connection from said amplifier through a capacitor to said common output, a source of modulating variable voltage, a connection from the modulating voltage source to said gate, circuit means connected across said transistor for obtaining the average of the instantaneous voltages on said source and drain and means including a series capacitor for applying said average of said voltages to said gate.
 3. The system called for in claim 2 in which the circuit means for obtaiNing the average of the instantaneous voltages comprises a pair of equal value resistors in series connected between the transistor source and drain and a tap at the common point between said resistors. 